1. Field of the Invention
The invention lies in the field of interferometric optical sensors and more particularly non contact sensors.
Recent developments in fiber optics have led to a number of applications that use fiber optic cables as a means of delivering light to interferometric transducers and fiber optic components to replace bulk optic components to transmit coherent light. Measurands such as temperature, vibration, pressure, and displacement have been demonstrated.
2. Prior Art
Many traditional bulk optic interferometers have been realized with equivalent fiber optic paths as well as systems that use fiber optics as the conduit for coherent light and external (extrinsic) interferometers. A number of non interferometric, amplitude modulation schemes have also been demonstrated. Cost is an important factor in all applications and fiber optics tends to be used only when there is a particular technical justification, such as immunity to electromagnetic interference, sensitivity and potential for multiplexing over long distances. Fringe counting and phase/quadrature signal processing for spatial fringes are well known as are homodyne demodulation systems for single temporal fringes. Laser diodes are commonly used as coherent sources as they are small and potentially low cost.
A disadvantage of laser diodes is that they are sensitive to external cavity feed back which is manifest as mode hopping and wavelength jitter. In the past, optical isolators have been used to prevent feed back, but optical isolators tend to be expensive. External mode locking using Fabry Perot cavities has been used to stabilize laser diodes but this makes wavelength modulation very difficult. In many cases the component count and cost of interferometric optical sensors is high. Many require complex demodulation techniques, have a large number of complex optical components and are expensive compared to traditional electromechanical sensors.